DESCRIPTION: The proposed experiments will use c-fos activation to assess the action of D1 and D2 receptors on neurons of the striatopallidal pathway. The first aim will determine whether intrastriatal injection of D1 and D2 agonists act in the striatum to enhance expression of fos in the globus pallidus and subthalamic nucleus intact and dopamine-denervated animals. The second aim will characterize the populations of globus pallidus neurons whose fos expression is affected by dopamine agonists or antagonists. The third aim is the characterization of the D2-mRNA containing cells in the globus pallidus, according to their projection patterns and parvalbumin immunostaining. The last aim will compare D2-mRNA containing pallidal neurons and those not containing the receptor mRNA in their response to D2 or combined D1+D2 agonists. These experiments are intended to elucidate the processes underlying the supersensitivity to dopamine and its agonists that occurs after dopaminergic denervation.